Astrophile: The climate-shaping supervolcanoes of Mars

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Objects: Martian supervolcanoesOutput: 4600-7200 cubic kilometres of lava and ash each over their lifetime

The road trip will last two years, and you can't pull over to stretch your legs or buy snacks. But the highland plains of Arabia Terra are sure to thrill any nature buff brave enough to make the trek to Mars. In addition to steep-walled craters and layered buttes, visitors will be able to explore the calderas of long-dead supervolcanoes.

These behemoths would have produced the most explosive eruptions ever seen on the Red Planet. Depending on when they were active, the volcanoes could have shaped the planet's climate and ability to host life.

Previous guidebooks might have pointed you to the slopes of Olympus Mons, the largest known volcanic peak in the solar system. But it coughed up lava relatively slowly, achieving its height over millions of years as fresh stuff was layered on top of older deposits.

"The types of volcanoes we're discussing now are fundamentally different," says Jacob Bleacher at the NASA Goddard Space Flight Center in Greenbelt, Maryland. "We see these calderas on the surface of Mars but not on top of a big structure like Olympus Mons. That means not a lot of material piled up at the vent, so the material must have been transported away."

Irregular craters

Bleacher and his colleague Joseph Michalski at the Planetary Science Institute in Tucson, Arizona, argue that irregular, crater-like features in a region called Arabia Terra are the calderas of supervolcanoes, akin to the one that shaped Yellowstone National Park in the US. On Earth, such supervolcanoes are hot spots capable of releasing at least 1000 cubic kilometres of lava and ash in a single, explosive outburst. The plumes of material shoot sky-high and spread across vast areas, leaving bowl-like calderas in an otherwise flat landscape. The last supervolcano eruption at Yellowstone 642,000 years ago released around 1000 cubic kilometres of material and covered roughly half of the continental US in ash.

Scouring orbital images, Bleacher and Michalski studied the Arabia Terra depressions and ruled out more familiar explanations. The features are probably not impact craters, for example, in part because they are missing surrounding haloes of ejected material.

Their sizes, depths and structures are good matches for supervolcano calderas. Although they can't yet say how many times each supervolcano blew, the volume of missing material suggests that an average Martian caldera could have spat out between 4600 and 7200 cubic kilometres of lava and ash in total.

Sulphurous rocks

The find could help explain a mystery about Arabia Terra. Until now, it was thought that the region was volcano-free, since it has no obvious peaks. But it also boasts thick layers of sulphur-bearing material, which probably has a volcanic origin, and there is too much for it to have been blown over from other volcanic zones. Local supervolcano eruptions would do the trick, the authors say.

On Earth, the effect of similar eruptions on climate has proved controversial. But Bleacher and Michalski think supervolcanoes on early Mars could have influenced that planet's climate and potential habitability. The key is timing. The team can't yet put ages on the calderas, other than noting that the surrounding rocks are at least a billion years old.

"These volcanoes we think could have been active back in the time period when the transition from warm, wet Mars to cold desert Mars was occurring or under way," says Bleacher. "But when that transition happened is not well constrained."

NASA's next Mars mission, an orbiter called MAVEN, is due to launch in November to study the Martian atmosphere and try to piece together what it was like in the past. Meanwhile, Bleacher says that better geological mapping and other techniques can help pin down the calderas' ages, offering better insight into the impacts of supervolcano eruptions.

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The Red Planet's answer to Yellowstone National Park (Image: NASA/JPL/MSSS/Google)